[time-nuts] DMTD - analog multiplier vs. diode mixer ?

Attila Kinali attila at kinali.ch
Sat Jan 9 20:25:23 UTC 2016

On Sat, 9 Jan 2016 10:19:05 +0800
Li Ang <lllaaa at gmail.com> wrote:

>     In some article, I see people use a D-flipflop to sample the input
> signal with reference clock. When you want implement a mixer what's the
> difference between D-flipflop and XOR gate?

A D-Flipflop is a rather weird mixer. I have not done the calculation,
but i'm pretty sure that the output is not exactly what you'd expect
it from a normal mixer (namely having half the energy at the frequeny
difference and half at the sum). 

An XOR gate on the other hand, produces a very nice spectrum, given
you input two clean square wave signals.

> Acorrding to my understanding,
> to multiply 1bit with another, I should use an AND gate, right?

If you think of the signals as digital in the computational sense,
with "high" representing "1" and "low" representing "0" then yes.
But in signal theory, it's more appropriate to think of signals
as "high" representing "+1" and low representing "-1".
In the latter case, the XOR is the multiplicative element, and not
the AND gate.

>     When you refer high speed CMOS XOR gate, do you mean 74LVC1G86?

Generally speaking: Faster CMOS better than slower CMOS in terms of phase noise.
(Though, I have yet to see actual measurements of this)

Single gate chips better than multi gate chips.
(no interference through the power supply of the different sub-parts)

				Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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